Understanding the pathways contributing to cancer pathogenesis is a critical step in developing targeted therapies and monitoring their efficacy. The p53 tumor suppressor pathway is inactivated in a majority of human cancers. p53 is a multifunctional protein that regulates gene transcription and has been implicated in the control of a number of cellular processes including differentiation, apoptosis, growth arrest, and DNA repair. Because the majority of cancers are epithelial in origin, we are exploring the mechanism(s) of tumor suppression by p53 and the cross-talk between p53 family members and other regulatory pathways using murine models of epithelial carcinogenesis. Using transgenic mice expressing a transactivation-deficient p53 mutant under control of the endogenous promoter, we have established that the ability of p53 to bind DNA and transactivate gene transcription is critical to its ability to suppress malignancy in ras-induced carcinogenesis. The contribution of p53-mediated gene transcription to other stress related responses is currently under study. p53 related proteins p63 and p73 share sequence homology with p53 in the transactivation, DNA binding, and oligomerization domains. The p63 and p73 genes are expressed as multiple isoforms displaying both overlapping and opposing functions. p63 expression is restricted to epithelial tissues and the gene is amplified in several human cancers. We have established that overexpression of a particular p63 isoform, DNp63alpha, as occurs in human cancers, blocks morphological and biochemical differentiation of keratinocytes, along with the associated growth arrest. Furthermore, while this isotype is known to block p53 mediated gene transcription, we have shown that it can also transactivate p53 responsive promoters. This positive gene regulatory activity is independent of p53 and appears to be specific for epithelial cells, suggesting a unique role for this isoform in epithelial biology consistent with its restricted expression pattern. Microarray studies have confirmed this activity and have suggested endogenous downstream targets. We are further evaluating the function and mechanisms of action of p53, related proteins, and downstream effectors in epithelial growth regulation and carcinogenesis.